In vehicles, and especially for heavier vehicles such as trucks, an auxiliary gearbox, also called a range gearbox, is often connected to a main gearbox to double the number of gears. Such an auxiliary gearbox usually includes a planetary gear, which has a low gear and a high gear, whereby the main gearbox shift facilities can be divided into a low range gear position and a high range gear position. In low range gear, a downshift takes place through the planetary gear, and, in the high range gear, the gear ratio is 1:1 in the planetary gear.
The range gearbox is usually provided between the main gearbox and a propeller shaft coupled to the drive wheels of the vehicle. The range gearbox is accommodated in a gearbox housing and comprises an input shaft coupled to the main gearbox and an output shaft coupled to the propeller shaft. Between the input shaft and the output shaft, the planetary gear is disposed. The planetary gear usually comprises three main components, which are rotatably arranged relative to each other. The three main components include a sun gear, a planet carrier with planet gears, and a ring gear. With knowledge of the number of teeth of the sun gear and the ring gear the relative speed of the three components can be determined during operation. In a range gearbox, the sun gear can be rotatably connected to the input shaft, a number of planet gears engage said sun gear and are rotatably mounted on the planet carrier, which is fixedly connected to the output shaft, and an axially displaceable ring gear surrounds and engages the planet gears.
In a known range gearbox, the low range gear position and the high range gear position are obtained by displacing the ring gear axially between the low range gear position, in which the ring gear is rotationally locked relative to the gearbox housing, and the high range gear position in which the ring gear is rotatable relative to the gearbox housing where the ring gear, the planet gears and the sun gear rotate as a common unit. The planetary gear comprises two coupling rings arranged on each side of the ring gear and two synchronizing rings arranged on each side of the ring gear. The synchronizer rings are arranged to provide a synchronous shift.
The document WO0155620 discloses a synchronization device in a planetary gear. The planetary gear includes a sun gear, a planet carrier and a ring gear. The sun gear is rotatably connected with the input shaft. A number of planet gears engage the sun gear and are rotatably mounted on the planet carrier, which is connected to the output shaft. An axially displaceable ring gear surrounds and meshes with the planet gears. Low and high gear are obtained by displacing the ring gear axially between the low range gear and the high range gear.
These synchronization means are subject to wear and are costly to repair. If the range gearbox is to transmit a large torque, the synchronization devices must be enlarged, which results in increased weight, increased space requirements and an increased moment of inertia.
There are range gearboxes in which the synchronization devices are replaced with coupling sleeves provided with splines. By controlling the transmission to synchronous speed between the two components to be connected it is possible to displace axially the coupling sleeve along the two components in order to connect the two components. When the components are to be detached, the transmission is controlled so that torque balance occurs between the components and the coupling sleeve is not transmitting torque. It then becomes possible to move the coupling sleeve along the components in order to disengage them from each other.
The document U.S. Pat. No. 6,196,944 shows a planetary gear comprising a sun gear, a planet carrier with planet gears and a ring gear. The sun gear may be connected to the input shaft by means of a coupling sleeve in a low range gear position and disengaged from the input shaft in a high range gear position. In the high range gear position the input shaft is connected to the planet carrier by means of the same coupling sleeve. The ring gear is firmly connected to a gearbox housing. The known planetary gear is arranged in an auxiliary gearbox, having only two gear positions.
The document U.S. Pat No. 6,196,944 shows a gearbox for motor vehicles comprising a planetary gear comprising a first and a second sleeve acting on the planet carrier, the ring gear, the gearbox housing and the output shaft. The first and second sleeves are controlled as one connected unit.
A gearbox provided with a double clutch system (also known as dual clutch system) comprises two or more input shafts, each connected to a clutch. The power from an energy source such as an internal combustion engine can be guided from one of the input shafts to the other by means of the clutch system, so that the power flow can be guided from one transmission shaft to a transmission shaft running in parallel, via gears meshing together.
Document US2014/0038769 A1 discloses a transmission using two transmission shafts running on axes parallel to each other, which can be coupled together. The transmission comprises a range gear.
However, when connecting a range gearbox to a main gearbox provided with a double clutch system it is difficult or even impossible to find a power flow through the transmission so that shifting gears can take place without an interruption of torque. An interruption of torque takes place in known gearboxes specially when shifting from a low range gear position to a high range gear position. A gearbox in which shifting of gears can take place without interruption of torque makes it possible to shift gears very fast, save fuel and provide comfort to the driver and the passengers of the vehicle in which the gearbox is arranged.
The space available for the powertrain in a vehicle is often limited. If the powertrain comprises several components, such as a combustion engine, a gearbox and a range gearbox, the design must be compact. At the same time, the components of the powertrain must be designed with dimensions that can absorb the necessary forces and torques.
A large number of gear steps are required in certain types of vehicles, in particular in heavy duty vehicles such as trucks and buses. In this case, the number of components of the gearbox increases, and the gearbox also must be dimensioned such that it can absorb large forces and torques, which arise in such heavy vehicles. Consequently, the size and the weight of the gearbox increase.
High demands are placed also on the reliability and the dependability, for the components of the powertrain. Wear influences the reliability and the lifetime of the gearbox.